3.5 Voltage Regulation Applications
Diodes have so many wonderful properties that there are a lot of applications for them that might now seem obvious. We have seen them used as rectifiers, but since they have the property that their voltage stays nearly constant while their current changes, we can take advantage of this to construct low voltage regulators. Take a look at Figure 3.43; here we see a circuit with 3 diodes in series, a current limiting resistor R, along with a load resistor RL, attached across the 3 diodes in series. Now, let’s apply an input voltage Vin from 5V to 12V; no matter what voltage we apply to the input Vin, there is going to be conduction in the diode branch. The 100 ohm resistor R limits the maximum current to Vin/100, but other than that we will get conduction. Since the diodes are conducting (hard) we will see a 0.7V drop across each for a total of 2.1V, so without a load we will see 2.1V at Vout no matter what the input voltage. Now, if we apply a load, say 1000 ohm, what happens? The current It will split into two currents It = Id + IL, IL = 2.1V/1000Ω = 2.1mA, and as long as there is still enough voltage to keep the diodes conducting, everything will be fine.
Figure 3.43 Diode based voltage regulator.
Again, this kind of regulation isn’t very stable and you have to series up a lot of diodes, and voltage drops over the diodes do change as you push more current through them, but for simple applications it works great. For example, if you have a device that needs 4V rather than 5.0V then you simply put a couple diodes that have a forward drop of 0.5V each in the current range you are going to draw and place them in front of the device. Presto, you have a lower voltage supply that is reasonably stable, and as long as you don’t have a lot of current needs for the device then the voltage will stay reasonably stable. Figure 3.44 shows this design.
Figure 3.44 Two-diode-based 4V voltage regulator.
3.5.1 Zener Diodes
The last type of diode I want to mention is a zener diode. These diodes look like normal diodes (maybe a little smaller), but have a slightly different schematic symbol as shown in Figure 3.45. Zener diodes are actually used in the reverse direction rather than the forward direction. The action of a zener diode is based on the fact that the resistance of a zener is inversely proportional roughly to the current driven through it, thus the voltage across it for a small range of currents stays constant. Thus it’s a regulator of sorts; you feed a zener from a higher voltage source in your circuit through a resistor and then create a voltage divider. As long as the zener current is met and the current or loads stay within range, you will have regulation.
Figure 3.45 Zener diode symbol.
Take a look at Figure 3.46: Here is a typical zener voltage regulator; Vin must be larger than the zener voltage you are trying to regulate to, but not much larger. Referring to the figure without a load, the resistor R sets up a current for the zener to operate and the zener drops exactly 5.1V (this is a 5.1V zener); then as a load is attached, as long as the load current through the zener doesn’t change too much, the zener will remain at 5.1V. Thus the system regulates. The details of the zener operation are based on an internal dynamic resistance of the zener, referred to as Rdyn; changes in the current through the zener result in changes in the voltage over it. With this knowledge we can write a simple modeling equation for the circuit as shown below:
- Vout – Vin = I * R
Now, if there is a change in Vin then there will be a change in Vout and finally a change in I. Writing the equation in terms of incremental differences we get
- ΔVout – ΔVin = ΔI * R
Rearranging a bit
- ΔVout = ΔVin + ΔI * R
Figure 3.46 Zener diode based voltage regulator.
Also, since we know that the internal resistance of the zener changes, there is a voltage divider action, so we can also write
- ΔVout = ΔVin * Rdyn / (R + Rdyn)
That is, the voltage drop over the zener is the change in input voltage multiplied by the voltage divided by the internal dynamic resistance of the zener and the resistor R.
You can use the mathematical model based on the dynamic resistance, or you can use a different approach, which is what I suggest. Simply set your zener current to a value and then based on the change in current, you can review the voltage of the particular zener you are working with and then see if the change in voltage relative to the change in current (the regulation) is acceptable. That’s what datasheets are for.